1. Field of the Invention
This invention relates to a coating formulation useful in forming a conductive coating film for its arrangement between a collector and an electrode active material layer (hereinafter also referred to as “electrode layer”), and also to a conductive coating film, a member for an electrode plate, the electrode plate for an electricity storage device, and the electricity storage device, all of which can be obtained using the coating formulation. More specifically, the present invention is concerned with a coating formulation capable of forming between a collector and an electrode layer a conductive coating film which is excellent in electrolyte resistance and oxidation resistance and can enhance the adhesion between the collector and electrode layer, and also with a conductive coating film, a member for an electrode plate, the electrode plate for an electricity storage device, and the electricity storage device, all of which can be obtained using the coating formulation.
2. Description of Related Art
A wide variety of chargeable and dischargeable batteries (storage devices) have been put to practical use as power sources for mobile equipment such as cellphones and notebook-size personal computers. Among these, commonly used are lithium ion secondary batteries which are light in weight, high in voltage and large in capacity, electric double-layer capacitors which are good in charge-discharge rate characteristics, and the like. Batteries for power applications such as vehicle-mounted batteries are, however, required to output or input a large current in a short time upon starting or stopping, so that conventional batteries are not suited for their high internal resistance and practically-insufficient charge-discharge cycle characteristics at high charge-discharge rate. Moreover, these conventional batteries are not sufficient in charge-discharge capacity from the viewpoint of cruising distances, and considering from the viewpoint of safety, are not sufficient either in the adhesion between electrode active material layers and metal collectors.
As one of reasons for which a conventional battery cannot exhibit sufficient characteristics as a battery as described above, it is possible to mention that the resistance value between its metal collector and active material layer is high and the adhesive force between its metal collector and active material layer is insufficient. As a measure for lessening these problems, a method has been proposed, which reduces the resistance at an interface between a metal collector and an active material layer and improves the adhesive force between them by providing the metal collector with a conductive coating layer (coating layer) and forming an active material layer on a surface of the coating layer (Patent Documents 1 and 2).
Especially, a method has been proposed, which uses, as a binder, a polyvinyl alcohol modified with silanol groups which chemically bond to hydroxyl groups on a surface of a metal collector as shown by the below-described equation (A) (Patent Documents 3 and 4). With this method, however, practically-sufficient charge-discharge cycle characteristics cannot be achieved at a high charge-discharge rate for batteries, which are intended for power applications, such as vehicle-mounted batteries. In particular, this method is insufficient for the prevention of deteriorations in battery characteristics when a long-term charge-discharge cycle test or high-temperature shelf test is conducted.
wherein R10 represents H or —COCH3, and R11 and R12 represent H or an ethyl group.
A method has also been proposed, which coats a conductive composition after treating a surface of a collector beforehand with a silane coupling agent or the like (Patent Document 5). The use of this method is, however, accompanied by a problem in that the resulting battery is provided with an increased internal resistance value, and hence, with a lowered charge/discharge rate because an active material layer is formed via an insulating layer although the insulation layer is thin.
To cope with these problems, a method has been proposed, which uses a conductive composition containing polyvinyl alcohol, a silane coupling agent, a polycarboxylic acid capable of forming ester bonds with hydroxyl groups on a metal surface of a collector and those of the polyvinyl alcohol and with silanol groups of the silane coupling agent, and a conductive aid (Patent Document 6). This method which makes use of the polycarboxylic acid can further improve the adhesive force of an active material layer to the collector, and therefore, can further improve the charge-discharge characteristics at a high charge-discharge rate over the conventional method which uses polyvinyl alcohol alone or the conventional method which uses a silanol-modified polyvinyl alcohol. This method is, however, not sufficient in avoiding deteriorations of battery characteristics after the battery is charged and discharged in multiple cycles over a long term or is left over in a charged state at high temperatures.
Further, positive electrode plates in a lithium ion secondary battery are placed under extremely strong acidic conditions. On the other hand, its negative electrode plates are placed under extremely strong reducing conditions. Concerning the coating films to be formed on the surfaces of its collectors, deteriorations and breakages caused by these severe conditions have, therefore, also arisen as problems. There is, hence, an outstanding desire for the development of coating films equipped with high oxidation resistance.
Furthermore, the conventional batteries and capacitors are also accompanied by the problems of the adhesion failure between electrode layers and collectors (substrates) and the high resistances at the interfaces between electrode layers and substrates as described above. Various coating formulations have been proposed. Conductive coating films formed with these coating formulations may bring about improvements in adhesion failure, but are still insufficient in electrolyte resistance and oxidation resistance. It is, therefore, the current situation that the resistances between electrode layers and collectors have increased further and the problems have not been resolved.